Circularly polarized antenna



March 19, 1957 Q s uc El'AL 2,786,199

CIRCULARLY POLARIZED ANTENNA Filed May 12, 1955 INVENTORS RAYMOND H.STRAUCH.

JEAN C. MENU- BY WM, A ms.

ROLAND A. ALLEIZARDI- United States Patent 2,786,199 CIRCULARLYPOLARIZED ANTENNA Raymond H. Strauch and Roland A. Allezard, Paris, andJean C. Menu, Ris-Orangis, France Application May 12, 1955, Serial No.507,541 I Claims priority, application France May 15, 1954 2 Claims.(Cl. 343857) This invention relates to a circularly polarized microwaveantenna having an axial radiation pattern.

It is known to construct circularly polarized microwave antennae byarranging, in the same plane, N antenna elements having approximately aquarter-Wavelength and making between themselves angles which are equalto degrees and feeding each of these elements with waves havingrespective phase-differences which are also equal I0 degrees. FrenchPatent No. 1,032,147, of February 7, 1951, for a Circularly PolarizedAntenna, and British Patent No. 567,201, of the March 5, 1943, provideantennae of this type, which comprise two crossed dipoles excited inquadrature, and United States Patent No. 2,480,182 of August 30, 1949,for an Antenna, describes an antenna of the same type comprising threequarter-wavelength elements spaced apart by 120 and driven by waveshaving respective phase-shifts of 120.

In an ideal circularly polarized antenna, the end of the radiatedelectric-field vector at a point located on the axis of the antennadescribes a polarization circle. At a point the direction of which makesan angle 1' with the axis of the antenna, the end of the electric vectordescribes an ellipse of polarization having a ratio, between the axes,of

minor axis major axis in certain applications, it is necessary that thefield remains circularly polarized even along directions oblique withrespect to the antenna pattern axis. However, polarization does notremain circular out of the axis for large values of the obliquity i whenthe antenna is formed by a discreet number of quarter-wavelengthelements distributed equiangularly.

The object of the present invention is to construct a circularlypolarized antenna for microwaves, the radiating element of which is anelement which has a revolution symmetry about the axis of the antennaand which gives a circularly polarized field in a wide beam angle.

According to the invention, the radiator of the antenna. is a flatcircular disc which is excited at N points, that are distributed atequal distances over a circle drawn on one of the faces of the disc, bywaves having respective phase-differences which are equal to the angulardistances apart between the points of excitation on the said circle.

Other objects and features of the invention will appear on reading thedetailed description of an example of embodiment which will now be givenwith reference to the accompanying drawing in which Fig. l is an axialsection of the antenna and Fig. 2 shows curves giving the ratio r =cos iPatented Mar. 19, 1957 in terms of the angle i between a given directionand the pattern axis.

The antenna is in the general form of a metallic cylinder 1 which isterminated at the radiating end by a dome 2 of dielectric material. Itis fed through a coaxial line 3, to which a wave having a wavelength isapplied. The internal conductor 4 of this line terminates in a cavity 5in the form of a probe which couples the line with the cavity.

Leading into the cavity 5 are N coaxial lines which are distributedequiangularly. In the example described, it is assumed that N :4, andtwo lines only, 6 and 8, are seen in the figure. The internal conductorsof these lines, namely 7 and 9 respectively, pass into the cavity 5where they constitute coupling probes.

The diameter of the cavity 5, that of the coupling probes 4, 7, 9 andthe characteristic impedances of the input line 3 and of the outputlines 6, 8 are conveniently fixed in order that the energy arrivingthrough the input line should be entirely transmitted to the outputlines.

The coaxial lines 6, 8 are filled with a dielectric substance having aconstant e, on a length lp, for the line of order 2 given by:

In the figure it has been assumed that the coaxial line 8 is entirelyfilled with dielectric material and that, in the coaxial line 6, thedielectric material is arranged in the form of pieces 14, all but one ofwhich has a length which is a multiple of a quarter of the wavelength inorder to eliminate reflection.

in front of the antenna, the external conductors are short-circuitedbetween each other by a plate in which are bored four holes and theinternal conductors 7, 9', 10' extend into a chamber 11 and areconnected to the disc 12 by screws 13 for example.

The length and the diameter of the conductors 7', 9, 1G which serve asimpedance-matching transformers, are determined by experiment for thepurpose of transmitting all the energy arriving through the coaxiallines 6, 8 to the disc 12. The circle, along which the points ofconnection between the conductors 7', 9, 10 are distributed, and thedisc 12 are such that the perimeter of a diametrical section of thedisc, reduced by the diameter of this circle, is approximately equal tohalf a wavelength.

A metallic ring 15, which is screwed to the cylinder 1, eliminatesrearward radiation.

Referring now to Fig. 2, the ratio r between the minor axis and themajor axis of the polarization ellipse is given in terms of the angle iin two planes passing through the antenna axis. Many curves have beenset forth by applicants and curves 16 and 17 are respectively the bestand the worst of said curves i. e. the curves for which the ellipticityof the polarization ellipse for a given obliquity is the nearest of andthe farthest from unity. It may be seen from line 18 that, if a ratio rbetter than is required from the antenna, this value is obtaincd in; anangle of approximately 40 on each side of the pattern axis,

What we claim is:

l. A circularly polarized microwave antenna comprisa: fiat: circulardisc having a frontv radiation face and a rear feed face, a singlegeneral coaxial feed line, a plurality of; particular coaxial feed linescoupled with said single line having outer and inner conductors, all

the outer conductors being connected between each other before the discand the inner conductorsv being connected to the rear faceof the discatpoints located over a feedcircleidrawn onsaid-faceand distributed atequal distances on said circle, and dielectric material plugs inserted:in

Rafmences Cited. in the file at this patent LJNSTED' STATES PATENTSLindenblad May 13, 1952 Albano Sept. 14, 1954

